129 research outputs found
STM Spectroscopy of ultra-flat graphene on hexagonal boron nitride
Graphene has demonstrated great promise for future electronics technology as
well as fundamental physics applications because of its linear energy-momentum
dispersion relations which cross at the Dirac point. However, accessing the
physics of the low density region at the Dirac point has been difficult because
of the presence of disorder which leaves the graphene with local microscopic
electron and hole puddles, resulting in a finite density of carriers even at
the charge neutrality point. Efforts have been made to reduce the disorder by
suspending graphene, leading to fabrication challenges and delicate devices
which make local spectroscopic measurements difficult. Recently, it has been
shown that placing graphene on hexagonal boron nitride (hBN) yields improved
device performance. In this letter, we use scanning tunneling microscopy to
show that graphene conforms to hBN, as evidenced by the presence of Moire
patterns in the topographic images. However, contrary to recent predictions,
this conformation does not lead to a sizable band gap due to the misalignment
of the lattices. Moreover, local spectroscopy measurements demonstrate that the
electron-hole charge fluctuations are reduced by two orders of magnitude as
compared to those on silicon oxide. This leads to charge fluctuations which are
as small as in suspended graphene, opening up Dirac point physics to more
diverse experiments than are possible on freestanding devices.Comment: Nature Materials advance online publication 13/02/201
Vitamin D (steroid hormone) and the nervous system diseases (literature review)
The present review aims to summarize the activities of vitamin D effects on the nervous system and to clarify a vitamin D role in brain diseases, in the pathogenesis or as a serum biomarker for the disease development and severity. Objective: correlation between chronic nervous system diseases and vitamin D level. Methods: a literature research in PubMed and in Russian electronic resources by keywords: vitamin D, brain diseases, chronic nervous system diseases. Results. Vitamin D as a neurosteroid hormone stimulates cerebral activity in both adult and embryonic brain regulates the activity of neural circuits which are responsible for locomotor, reward-dependent and emotional behavior. Patients with autism spectrum disorders, schizophrenia, Alzheimer disease, multiple sclerosis, Parkinson disease and sleep disorders have been shown to have low level of vitamin D. Discussion. Data are controversial, a further study of vitamin D hypovitaminosis significance is essential for the nervous system chronic diseases manifestation and evaluation of the vitamin D dietary supplement efficiency in patients with the nervous system pathology
Water-Gated Charge Doping of Graphene Induced by Mica Substrates
We report on the existence of water-gated charge doping of graphene deposited
on atomically flat mica substrates. Molecular films of water in units of ~0.4
nm-thick bilayers were found to be present in regions of the interface of
graphene/mica hetero-stacks prepared by micromechanical exfoliation of kish
graphite. The spectral variation of the G and 2D bands, as visualized by Raman
mapping, shows that mica substrates induce strong p-type doping in graphene,
with hole densities of {-2}$. The ultrathin water
films, however, effectively block interfacial charge transfer, rendering
graphene significantly less hole-doped. Scanning Kelvin probe microscopy
independently confirmed a water-gated modulation of the Fermi level by 0.35 eV,
in agreement with the optically determined hole density. The manipulation of
the electronic properties of graphene demonstrated in this study should serve
as a useful tool in realizing future graphene applications.Comment: 15 pages, 4 figures; Nano Letters, accepted (2012
Electronic properties of bilayer and multilayer graphene
We study the effects of site dilution disorder on the electronic properties
in graphene multilayers, in particular the bilayer and the infinite stack. The
simplicity of the model allows for an easy implementation of the coherent
potential approximation and some analytical results. Within the model we
compute the self-energies, the density of states and the spectral functions.
Moreover, we obtain the frequency and temperature dependence of the
conductivity as well as the DC conductivity. The c-axis response is
unconventional in the sense that impurities increase the response for low
enough doping. We also study the problem of impurities in the biased graphene
bilayer.Comment: 36 pages, 42 figures, references adde
Snap-through instability of graphene on substrates
We determine the graphene morphology regulated by substrates with herringbone
and checkerboard surface corrugations. As the graphene/substrate interfacial
bonding energy and the substrate surface roughness vary, the graphene
morphology snaps between two distinct states: 1) closely conforming to the
substrate and 2) remaining nearly flat on the substrate. Such a snapthrough
instability of graphene can potentially lead to desirable electronic properties
to enable graphene-based devices.Comment: 13 pages, 4 figures; Nanoscale Research Letters, in press, 200
Compression Behavior of Single-layer Graphene
Central to most applications involving monolayer graphene is its mechanical
response under various stress states. To date most of the work reported is of
theoretical nature and refers to tension and compression loading of model
graphene. Most of the experimental work is indeed limited to bending of single
flakes in air and the stretching of flakes up to typically ~1% using plastic
substrates. Recently we have shown that by employing a cantilever beam we can
subject single graphene into various degrees of axial compression. Here we
extend this work much further by measuring in detail both stress uptake and
compression buckling strain in single flakes of different geometries. In all
cases the mechanical response is monitored by simultaneous Raman measurements
through the shift of either the G or 2D phonons of graphene. In spite of the
infinitely small thickness of the monolayers, the results show that graphene
embedded in plastic beams exhibit remarkable compression buckling strains. For
large length (l)-to-width (w) ratios (> 0.2) the buckling strain is of the
order of -0.5% to -0.6%. However, for l/w <0.2 no failure is observed for
strains even higher than -1%. Calculations based on classical Euler analysis
show that the buckling strain enhancement provided by the polymer lateral
support is more than six orders of magnitude compared to suspended graphene in
air
Evolution of Microscopic Localization in Graphene in a Magnetic Field from Scattering Resonances to Quantum Dots
Graphene is a unique two-dimensional material with rich new physics and great
promise for applications in electronic devices. Physical phenomena such as the
half-integer quantum Hall effect and high carrier mobility are critically
dependent on interactions with impurities/substrates and localization of Dirac
fermions in realistic devices. We microscopically study these interactions
using scanning tunneling spectroscopy (STS) of exfoliated graphene on a SiO2
substrate in an applied magnetic field. The magnetic field strongly affects the
electronic behavior of the graphene; the states condense into welldefined
Landau levels with a dramatic change in the character of localization. In zero
magnetic field, we detect weakly localized states created by the substrate
induced disorder potential. In strong magnetic field, the two-dimensional
electron gas breaks into a network of interacting quantum dots formed at the
potential hills and valleys of the disorder potential. Our results demonstrate
how graphene properties are perturbed by the disorder potential; a finding that
is essential for both the physics and applications of graphene.Comment: to be published in Nature Physic
Properties of Graphene: A Theoretical Perspective
In this review, we provide an in-depth description of the physics of
monolayer and bilayer graphene from a theorist's perspective. We discuss the
physical properties of graphene in an external magnetic field, reflecting the
chiral nature of the quasiparticles near the Dirac point with a Landau level at
zero energy. We address the unique integer quantum Hall effects, the role of
electron correlations, and the recent observation of the fractional quantum
Hall effect in the monolayer graphene. The quantum Hall effect in bilayer
graphene is fundamentally different from that of a monolayer, reflecting the
unique band structure of this system. The theory of transport in the absence of
an external magnetic field is discussed in detail, along with the role of
disorder studied in various theoretical models. We highlight the differences
and similarities between monolayer and bilayer graphene, and focus on
thermodynamic properties such as the compressibility, the plasmon spectra, the
weak localization correction, quantum Hall effect, and optical properties.
Confinement of electrons in graphene is nontrivial due to Klein tunneling. We
review various theoretical and experimental studies of quantum confined
structures made from graphene. The band structure of graphene nanoribbons and
the role of the sublattice symmetry, edge geometry and the size of the
nanoribbon on the electronic and magnetic properties are very active areas of
research, and a detailed review of these topics is presented. Also, the effects
of substrate interactions, adsorbed atoms, lattice defects and doping on the
band structure of finite-sized graphene systems are discussed. We also include
a brief description of graphane -- gapped material obtained from graphene by
attaching hydrogen atoms to each carbon atom in the lattice.Comment: 189 pages. submitted in Advances in Physic
Heterogeneous Nucleation of Protein Crystals on Fluorinated Layered Silicate
Here, we describe an improved system for protein crystallization based on heterogeneous nucleation using fluorinated layered silicate. In addition, we also investigated the mechanism of nucleation on the silicate surface. Crystallization of lysozyme using silicates with different chemical compositions indicated that fluorosilicates promoted nucleation whereas the silicates without fluorine did not. The use of synthesized saponites for lysozyme crystallization confirmed that the substitution of hydroxyl groups contained in the lamellae structure for fluorine atoms is responsible for the nucleation-inducing property of the nucleant. Crystallization of twelve proteins with a wide range of pI values revealed that the nucleation promoting effect of the saponites tended to increase with increased substitution rate. Furthermore, the saponite with the highest fluorine content promoted nucleation in all the test proteins regardless of their overall net charge. Adsorption experiments of proteins on the saponites confirmed that the density of adsorbed molecules increased according to the substitution rate, thereby explaining the heterogeneous nucleation on the silicate surface
sp-Electron Magnetic Clusters with a Large Spin in Graphene
Motivated by recent experimental data (Sepioni, M. et al. Phys. Rev. Lett.
2010, 105, 207205), we have studied the possibility of forming magnetic
clusters with spin S> 1/2 on graphene by adsorption of hydrogen atoms or
hydroxyl groups. Migration of hydrogen atoms and hydroxyl groups on the surface
of graphene during the delamination of HOPG led to the formation of seven-atom
or seven-OH-group clusters with S=5/2 that were of a special interest. The
coincidence of symmetry of the clusters with the graphene lattice strengthens
the stability of the cluster. For (OH)7 clusters that were situated greater
than 3 nm from one another, the reconstruction barrier to a nonmagnetic
configuration was approximately 0.4 eV, whereas for H7 clusters, there was no
barrier and the high-spin state was unstable. Stability of the high-spin
clusters increased if they were formed on top of ripples. Exchange interactions
between the clusters were studied and we have shown that the ferromagnetic
state is improbable. The role of the chemical composition of the solvent used
for the delamination of graphite is discussed.Comment: 22 pages, 1 table, 4 figures. Minor changes, few refs added. Accepted
to ACS Nan
- …